Optical modulators are generally devices which change (modulate) the intensity, phase, polarized state, wavelength/frequency, traveling direction, and the like of an optical signal which carries information by using, e.g., external electric, magnetic, mechanical, acoustic, and optical means. Of these devices, a device which modulates the intensity or phase of an optical signal by using an electric means is conventionally widely used from the viewpoints of the operation speed (operation bandwidth) and controllability.
Typical examples of the device which modulates the intensity of light are an electroabsorption modulator which modulates the intensity of light (transmits or absorbs light) which propagates in a material forming the device by controlling the light absorption coefficient of the material by applying an electric field to the device, and a Mach-Zehnder modulator which replaces a refractive index change of a device material caused by field application and a consequence phase change of an optical signal with an intensity change by using the interference effect of a Mach-Zehnder interferometer.
Although the electroabsorption modulator and Mach-Zehnder modulator use different physical phenomena as device operating mechanisms, the both are devices having an aspect as an optical device including optical signal input and output terminals and an aspect as an electric device including electrical signal input and output terminals, when they are regarded as devices which input a modulation electrical signal and outputs a modulated optical signal. The ratio of the modulation of the intensity of an optical signal described above, i.e., the extinction ratio, is one important performance index of the optical modulator from the viewpoint of the aspect as an optical device.
On the other hand, when the operation bandwidth of the optical modulator is taken into consideration, the aspect as an electric device should be noted. For example, the conventional electroabsorption modulator has an electrode structure in which the operation bandwidth of the device is limited by a CR time constant where C is a device capacitance as a lumped element and R is a load resistance. In this case, the device capacitance must be reduced to enlarge the operation bandwidth of the device. However, if, for example, the device length (the length in a direction in which an optical signal propagates) is shortened to reduce the device capacitance, the extinction ratio decreases. Also, if, for example, the device thickness (the length in a direction in which the electric field of an electrical signal is applied) is increased, the driving voltage increases.
It is, therefore, recently proposed to greatly alleviate the bandwidth limitation by the CR time constant described above by changing the electrode structure of the device from the lumped-element type to a traveling-wave type (distributed-element type). The traveling-wave type electrode structure is a structure in which an electrode for an electric signal (microwave) is formed into a distributed-element type transmission line such as a coplanar line or microstrip line, and this transmission line and an optical signal waveguide are formed parallel to each other. In this structure, the operation bandwidth of the device is presumably determined by a phase speed difference between an electrical signal and optical signal which propagate in the device, so characteristics over an extremely wide band can be expected. In effect, ultra-wide-band characteristics by which the 3-dB-down bandwidth of the E/O (Electrical-to-Optical) response is, e.g., 50 GHz or more is realized by the traveling-wave type electrode structure device.
As described above, the traveling-wave type electrode structure device propagates an electrical signal on a transmission line, and a transmission line generally has a characteristic impedance (Z0), so it is essential to match the characteristic impedance of the line with the impedance of a terminating resistor of an electrical signal driving system in order to efficiently transmit an electrical signal. A standard electrical signal driving system is a 50 -Ω system (i.e., the terminating resistor is 50 Ω).
When the traveling-wave type electrode structure optical modulator is regarded as an electric device having a transmission line, its characteristic impedance Z0 is typically about 25 Ω, so it looks like a low-impedance line from the 50-Ω driving system, and this produces impedance mismatching. If this impedance mismatching occurs, a portion of a modulation electrical signal as a microwave is reflected when the signal is input to the optical modulator, so the external input microwave is not efficiently supplied to an electrical/optical interaction region in the optical modulator. This consequently deteriorates the flatness of the frequency characteristic or decrease the 3-dB-down bandwidth of the E/O response.
As a method of improving the problem of impedance mismatching in the optical modulator, an arrangement in which, as shown in FIG. 30, an electrical signal line 3 which connects an electrical/optical interaction region 11 in an optical modulation unit 10 and an input terminating resistor 81 and an electrical signal line 4 which connects the electrical/optical interaction region 11 and an output terminating region 91 are high-impedance lines having a characteristic impedance Z0 of about 100 Ω is proposed (Electronic Letters 1st May 2003, Vol. 39 No. 9, pp. 733-735).
In this arrangement, when the low-impedance line (Z0 of about 25 Ω) of the optical modulation unit 10 and the high-impedance line (Z0 of about 100 Ω) connected in series with the low-impedance line is considered to be one device as a whole, the effective characteristic impedance can be regarded as the average value of the characteristic impedances of the individual lines, so impedance matching with the 50-Ω driving system is possible. Since this reduces an input reflection coefficient (S11) and output reflection coefficient (S22) with respect to the microwave, and the microwave input from the input terminating side of the electrical signal driving system (driver circuit) to the optical modulator is efficiently transmitted to the output terminating side of the electrical signal driving system, the efficiency of application of the microwave voltage to the electrical/optical interaction region in the optical modulator increases. As a consequence, it is possible to improve the flatness of the frequency characteristic of the E/O response.
As another method of improving the flatness of the frequency characteristic of the E/O response, an arrangement which makes the value of the output terminating resistor (load resistor) smaller than the value of the characteristic impedance of the optical modulator is proposed (e.g., Japanese Patent Laid-Open No. 11-183858). This method in which the value of the output terminating resistor is made different from the value of the characteristic impedance of the optical modulator intentionally produces impedance mismatching on the output terminating side of an electrical signal, and uses, as electrical signals to be applied to the electrical/optical interaction region in the optical modulator, not only an incident electrical signal from the driver circuit but also a reflected electrical signal generated by the impedance mismatching on the output terminating side. This makes it possible to control the profile (form) of the frequency characteristic of the E/O response, improve the flatness, and increase the 3-dB-down bandwidth.